A detector preamplifier is an optical receiver component utilized in a variety of applications including pulse oximetry. Pulse oximetry is a noninvasive, easy to use, inexpensive procedure for monitoring blood oxygen that has gained rapid acceptance in a wide variety of medical applications, including surgical wards, intensive care units, general wards and home care. A pulse oximeter sensor detects the light absorption characteristics of blood perfused tissue, and a corresponding monitor performs a blood spectral analysis based upon the sensor signal. By monitoring for decreases in the arterial oxygen supply, pulse oximeters reduce the risk of accidental death and injury.
FIG. 1 illustrates an analog portion of a pulse oximetry system 100 having a sensor 110 and a monitor 150. The sensor 110 has emitters 120 and a detector 130. The emitters 120 typically consist of a red LED (light emitting diode) and an infrared LED that project light through blood vessels and capillaries underneath a tissue site, such as a fingernail bed. The detector 130 is typically a photodiode positioned so as to detect light transmitted through or reflected from the tissue site. The signal current generated by the detector 130 is proportional to the intensity of the light emerging from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.
Also shown in FIG. 1, the monitor 150 has drivers 160, a preamplifier 200, signal conditioning 180 and digitization 190. The drivers 160 alternately activate the emitters 120. The preamplifier 200 provides an amplified detector signal for signal conditioning 180, which typically includes filtering and additional amplification. The digitization 190 performs an analog-to-digital conversion (ADC) of the conditioned detector signal. The resulting digitized signal is then analyzed by a digital signal processor (not shown) to determine oxygen saturation based upon the differential absorption by arterial blood of the two wavelengths projected by the emitters 120. A pulse oximetry signal processor is described in U.S. Pat. No. 6,081,735 entitled “Signal Processing Apparatus,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.
FIG. 2 illustrates a circuit for a conventional pulse oximeter preamplifer 200. Other names for this circuit are a current-to-voltage converter, photodiode amplifier and transimpedance amplifier. An operational amplifier (op amp) 210 provides the signal gain. The op amp 210 sets the output 216 so that the current flowing through the feedback impedance R4, C2 240 is equal to the current flowing through the input resistors R1, R2 220, R3 230 and the photodiode detector 130. The detector generated current creates a negative voltage at the op amp output 216. The preamp DC gain (output voltage/detector current) is approximately R4, and the high frequency response of the preamp is approximately Fcutoff=1/(2πR4C2).